The provision of steam to a turbine is typically accompanied by the transfer of heat between the steam and those components of the turbine coming into contact with the steam. Since most turbine components are formed from highly thermally conductive material, heat transfer can cause thermal distortion in various components due primarily to nonuniform heat transfer. Such deformation can be of two types: elastic which is recoverable upon release of the transferred heat, and plastic which is permanent.
One form of elastic thermal distortion can occur during shutdown transients in high pressure (HP) steam turbines, intermediate pressure (IP) steam turbines and combination HP-IP steam turbines. In particular, during shutdown transients seal rubs can occur in those turbines. Those familiar with HP, IP and HP-IP steam turbines will recognize that during shutdown transients relatively large temperature differentials can develop between the cover and base components of the outer cylinder. These temperature differentials can reach approximately 120.degree. F. Typically, the cover is hotter than the base causing the outer cylinder to hump, i.e. rise in the center and pivot at the support paws. Since internal stationary parts, i.e. inner cylinders and rings, are attached to the outer cylinder, they too move in relation to the outer cylinder. Since the rotor is supported independently of the outer cylinder it retains its normal configuration. As a result, seal clearances between rotating and stationary parts close in the base and open in the cover. This phenomenon is referred to as turbine humping. If the humping is severe enough undesirable rubbing will occur.
In the past, one approach to prevent turbine humping has been the use of heating blankets in an effort to reduce the temperature differential. Unfortunately such a solution is very costly.
In the development of the present invention, an investigation was made of the turbine humping phenomenon. It was discovered for certain types of HP-IP turbines that 33 to 50 percent of the humping was attributable to the support paws. Support paws are extensions or projections generally integrally formed at the ends of the outer cylinder for supporting the outer cylinder on the foundation structure. When the turbine humps, the support paws act as pivot points. The longer the paw, the greater the pivot action. It was also discovered that the support paws had a thermal gradient of approximately 150.degree. F. along its length. This thermal gradient contributes a second order effect to overall humping.
Some presently available steam turbines incorporate cover support paws which project straight off the cover portion of the outer cylinder and which are generally shorter in length. Unfortunately, cover support paws suffer from two problems. First, assembly and disassembly of turbines using such supports is complicated and thus more costly. Second, the horizontal joint bolting must support the weight load of the turbine as well as normally supported loads.
Another form of elastic thermal distortion that can occur in steam turbines results from thermal expansion and contraction of the outer cylinder and is referred to as thermal displacement. During thermal displacement in steam turbines where the support plane and the horizontal joint plane are separated a distance, portions of the stator assembly are displaced in relation to the rotor assembly effecting turbine operating efficiency and which can result in rubbing.
Consequently, a need still exists for a steam turbine which is free from humping without resorting to heating blankets, which is free from the drawbacks of cover support paws and in which thermal displacement is minimized.